The causes of rusting

Read our standard health & safety guidance

Lesson Organisation

This experiment will need to be set up in one lesson and then left for more than 3 days before being re-examined. It could be left set up for longer if necessary.

Apparatus and Chemicals

Per pair or group of students:

Eye protection
Test-tubes, 4
Cotton wool
Iron nails, 4
Test-tube rack
Rubber bung to fit a test tube
Forceps
Pen or other means of labelling test tubes

Calcium chloride (anhydrous granules) (Irritant) Students with sensitive skin should be offered gloves.
Cooking oil
Deionised water
Boiled deionised water (15 min boil) (see note 1)
Sodium chloride (table salt) (Low hazard)

Technical notes

Anydrous calcium chloride (Irritant) Refer to CLEAPSS Hazcard 19A
Sodium chloride (Low hazard) Refer to CLEAPSS Hazcard 47B

1 It is best if the deionised water is boiled eg in a kettle, as close to the start of the lesson as possible and supplied warm to the students. They could boil it themselves for 15 minutes in a beaker on a Bunsen burner, but whether this is advisable will depend on the class.

Procedure

HEALTH & SAFETY: Wear eye protection.

a Label the test tubes 1-4.

b About ¼ fill tube 1 with deionised water and add a nail.

c About ¼ fill tube 2 with boiled deionised water and add a nail. Carefully pour a little oil over the surface to prevent air from reaching the water.

d Mix some salt with some deionised water to make a solution. About ¼ fill tube 3 with this mixture and add a nail.

e Put a nail into tube 4 and add about 2 cm depth of anhydrous calcium chloride granules. These absorb water. Put a bung in this tube to prevent any further water from getting in.

f Leave for at least 3 days and then note any changes in appearance of the nails.

Teaching notes

You could ask students to tabulate which conditions are present or absent in each of the tubes.
• Tube 1 – water and air
• Tube 2 – water but no air (it is removed during boiling and the oil prevents any extra from dissolving in the water and reaching the nail)
• Tube 3 – water, air and salt
• Tube 4 – air, no water (the calcium chloride removes the water from the air and the bung prevents any extra from entering.)

They should see that the nails in tubes 2 and 4 do not rust. The nail in tube 3 rusts the most. From this they should be able to conclude that water and air (actually oxygen in the air) are essential for rusting. Salt can increase the rate of rusting. This can lead to a discussion about rust protection and methods which can be used to keep air and water away from the iron such as paint, grease and plastic coating.

Very simply, rusting is the reaction of iron with oxygen – but water is an important part of the process too. For fuller details of the reactions, see the website below.

Health & Safety checked, April 2008

Web Links

For details of the rusting reaction see: http://library.kcc.hawaii.edu/external/chemistry/everyday_corrosion.html

(Websites accessed April 2008)